Presently, induction motors are used as submersible motors in various applications, including driving submersible electric pumps in oil fields. In some applications, an induction motor is a single-phase alternating-current motor. But its three-phase model is used more frequently. The induction motor operation philosophy is based on the interaction between a rotating magnetic field that occurs when three-phase alternating current passes through the stator windings, and the current induced by the stator field in the rotor windings, which results in occurrence of mechanical forces that make the rotor rotate in the direction of the rotating field rotation, provided that the rotor rotation speed is lower than the field rotation speed. So, the rotor rotates asynchronously with respect to the field. The disadvantages of induction motors include a limited rotation speed control range and considerable reactive power consumption under the low-load conditions. Moreover, the speed control system is rather complicated due to the need to adjust the frequency of current, and the presence of three wires in the winding makes this adjustment a rather difficult process.
The above-mentioned problems could be solved by using a direct-current motor. Operation of direct-current motors is based on the interaction between current conductors and magnets, which results in occurrence of electromagnetic torque. These motors are characterized by a very compact design, which provides a high current density in the primary winding. Such machines operate in a wide rotation speed range and have a rather simple control system. However, the main disadvantage of these motors when used as submersible pump drives is that a short circuit occurs between the brushes and the collector when the motors are submerged into a fluid, particularly, into an electrically conducting fluid. This may cause damage to the brushes and/or to the collector.